It has been demonstrated that hydrogenated amorphous semiconductors is very useful in making various kinds of photo-sensing and detecting devices.
For photo-sensing and detecting purpose, it is necessary that the device should possess high sensitivity, short switching time, wavelength selectivity, high gain and high speed.
Recently, we have successfully fabricated an amorphous silicon homojunction bulk barrier phototransistor, with high gain (.about.12) and high speed (.about.30 .mu.s), but the functioning of the kind of phototransistors mentioned above is not enough. We need more efficient phototransistors to improve the efficiency of photo-sensing and detecting devices.
In order to improve the device performance, a new type of amorphous heterojuction phototransistor with the structure of ITO/n.sup.+ (a-S.sub.i C)/n-i-p(a-S.sub.i C)/i-n.sup.+ (a-S.sub.i)/Al is successfully fabricated by the plasma decomposition of an [(1-X)S.sub.i H.sub.4 +XCH.sub.4 ] gas mixture.
A heterojunction phototransistor with large bandgap at the emitter potentially possesses higher gain than a homojunction device, due to a carrier accumulation effect at the emitter-base junction, in which we have adopted a-S.sub.i C: H as a wide bandgap material.
This new phototransistor structure has a very thin a-S.sub.i C base (100 .ANG.), and a-S.sub.i C emitter which provide an effective barrier to accumulate more photo-generated holes at the base and therefore significantly improve the gain.